Thursday, June 18, 2015

Oral contrast for abdominal CT?

This material is educational, and is not intended to serve
as hospital protocol.

Mostly, it's here to serve a quick reference for myself, and for impromptu discussions with PAs and residents.

SUMMARY

Suspected SBO

Oral contrast is contraindicated
in suspected SBO.

IV contrast is preferred, but not required.

Suspected
appendicitis

Oral contrast is not required, but may be beneficial in extremely thin individuals (e.g. BMI
< 18), or non-obese children.

Suspected
diverticulitis

Oral contrast is not required

Unclear etiology

PO contrast is suggested by some experts in undifferentiated
abd pain if the patient

1)Is very thin (e.g. < 120 pounds, BMI <
18);

2)Has had a Roux-en-Y gastric bypass; or

3)Has and inflammatory bowel disease that could
produce a fistula

Discussion and References

SBO

Per the American
College of Radiology guidelines, in a patient with suspected SBO, PO
contrast is contraindicated. They explain that: “Oral contrast will
not reach the site of obstruction, wastes time, adds expense, can induce
further patient discomfort, will not add to diagnostic accuracy, and can lead
to complications, particularly vomiting and aspiration.”

Multiple studies have shown that the use of PO contrast does
not increase the accuracy of the diagnosis of appendicitis (references below).
An editorial in a
recent radiology journalconcluded that “routine
administration of oral contrast medium may not be necessary in the setting of
suspected acute appendicitis.” American
College of Radiology guidelines state that PO contrast “may not be
needed,” leave the decision to use PO contrast to “institutional preference.”

Some have suggested that PO contrast
might be needed in extremely thin individuals (e.g. BMI < 18). See the discussion below in "Unclear etiology."

A 2009 randomized study in the American
Journal of Radiology found that “nontraumatic
abdominal pain imaged using 64-MDCT with isotropic reformations had similar
characteristics for the diagnosis of appendicitis when IV contrast material
alone was used and when oral and IV contrast media were used.”

A 2011 retrospective study in the Journal of Surgical Research
found that oral contrast did not reach the cecum in 1/3 of patients.
Furthermore, “there appears to be no diagnostic compromise in those without
contrast in the terminal ileum.

A 2014 prospective study in the Annals of Surgery
concluded that: “Enteral contrast should be eliminated in IV-enhanced CT scans
performed for suspected appendicitis.”

Diverticulitis

The American
College of Radiology guidelines state that PO contrast “may be helpful
for bowel luminal visualization.” Despite this, they conclude that “regardless
of the [the use or lack of IV or PO contrast], the accuracy is high for
depicting findings of acute diverticulitis.”

A 2006 prospective study in Emergency Radiologylooked
at patients with undifferentiated abdominal pain in the ED; 1/5 of them had LLQ
tenderness, and diverticulitis was the second-most common suspected diagnosis. All
patients had CT scans with and without PO contrast; no IV contrast was used. Although there were discordant
interpretations between the (+) PO and (-) PO CT scans, the authors found that
a “significant portion of the discordance was attributable to interobserver
variability.” Thus, it would appear that a CT scan without either PO or IV contrast
could be accurate in the diagnosis of diverticulitis.

Unclear
etiology

Oral contrast may not be needed for undifferentiated
abdominal pain, as per the discussion of the study in Emergency
Radiology noted above.

However, in a 2012 study in Emergency
Radiology that examined the use of PO contrast in undifferentiated
non-traumatic abd pain, the authors excluded 2 groups of patients. They excluded all “subjectively thin”
patients, but did not provide cut-offs. They noted the research that suggests
this exclusion is unnecessary, and may be dropped in the future. They also excluded patients at high risk for
having an intra-abdominal fistula. Primarily, they targeted patients who had had
a Roux-en-Y gastric bypass, or who had IBD, disposing them to fistula
formation.

5 comments:

ACR recommendations you cite are for "high-grade SBO". For completeness, the ACR further says that for intermittent or low-grade SBO, "in contrast to a high-grade obstruction, oral contrast is not necessarily contraindicated and may add functional information."

As a radiologist (having seen one or two CT scans) I find the use of enteric contrast with CT to be often helpful in problem solving.